In both young animals and adults, an increase in intestinal blood flow and perfused capillaries occurs after feeding, and is generally considered to facilitate the absorption of nutrients. Increased blood flow may act by supplying additional oxygen required for the increased metabolism associated with active transport of nutrients, and aid passive absorption by enhancing nutrient removal from the interstitium. We wish to answer the following questions, which have not previously been addressed: Is the vascular response of the intestine to feeding compromised with increasing age? If so, does this compromise the absorptive capacity of the aged intestine? Initially, we propose to answer the first question by making the following quantitative measurements in jejunal tissue of 3 and 30 mo. old rats 2 hr before or after feeding: (1) oxygen consumption in the muscularis, and the submucosa-mucosa; (2) arterio-venous oxygen gradients in the lower portion of the villus; (3) morphometric indices of the total and perfused capillary network in the muscularis, the submucosa-mucosa and the lower and upper portions of the villus. Vascular competence in the aged intestine will be evaluated under fasting conditions and in response to feeding. Several methods that have not previously been applied to intestinal tissue will be used in this work. Perfused capillaries will be measured in photographs of 2Mu thick sections of intestine after the systemic injection of fluorescent dye into the anaesthetized rat. The sections will then be stained with alkaline phosphatase to visualize total capillary beds, and rephotographed. Regional oxygen consumption will be derived from microspectrophotometric measurements of oxygenated and deoxygenated hemoglobin in the blood vessels. Regional blood flow will be derived from measurements of microsphere accumulation in the same vessels. If old and young animals are found to differ in these parameters, we will try to correlate these changes with effects on nutrient absorption. Using a variety of pharmacological agents known to alter intestinal blood flow, the effect of altered blood flow on nutrient absorption, and the role of these factors in altering blood flow, can be assessed in a systematic way. The establishment of a casual relationship between nutrient absorption and intestinal oxygenation, and the use of factors effective in altering blood flow, could lead to the development of more effective therapeutic strategies for meeting the nutritional needs of elderly patients.